1. Field of the Invention
The present invention relates to an image displaying apparatus and a display controlling device for having an image displayed by using wavelength conversion laser light sources.
2. Description of the Related Art
Conventionally, projection-type image displaying apparatuses like projection televisions use a spatial modulation element called micro display to magnify and project a generated image. One example of the micro display is a device called Digital Micromirror Device® (DMD), which is an element that performs a spatial modulation process by changing the reflection angle of light by using a very small movable mirror. This element employs a method by which a gradation display is achieved through digital control.
In the gradation display method implemented through the digital control, a time period corresponding to one TV field (59.9 hertz [Hz]) is further divided into sections (each of which has a length of, for example, microseconds to milliseconds), so that the modulation process is performed by switching on and off the pixels in a micro display for each section time period. Because this switching method (called a “field sequential method”) applied to the pixels has an extremely high speed, the human eye recognizes a predetermined gradation level in each TV field, as a result of integration.
In recent years, many techniques for using laser light sources as light sources included in such image displaying apparatuses that employ the field sequential method have been proposed. Because laser beams have high directionality and high monochromaticity, when laser beams are used as the light sources, an advantageous effect is achieved where it is possible to provide an image displaying apparatus that has high image quality as well as a high luminance and a wide range of color reproducibility. To display an image in full color by using an image displaying apparatus that includes such laser light sources, it is necessary to use laser beams of which the light emissions are in the wavelength bands of at least the three primary colors (i.e., red [R], green [G], and blue [B]). Also, it is desirable to use laser light sources that are compact and have high electro-optic efficiency (i.e., high conversion efficiency). As the laser light sources, for example, a laser diode (LD) can be used for red (R), whereas Diode Pumped Solid-State Lasers (DPSSL) that emit wavelength conversion laser beams can be used for green (G) and blue (B).
These days, image displaying apparatuses have a tendency of having a larger screen and a higher luminance. Thus, the luminance level of the laser beam per module as described above is not sufficient. For example, to provide an image displaying apparatus having a projection luminous flux of 500 lumens (lm), it is necessary to use two or three high-output laser modules each having an average power of 3 watts (W). Thus, to provide an image displaying apparatus that uses laser light sources, it is an important goal to employ a plurality of laser modules in an optimal configuration while improving the luminance efficiency of each of the laser modules. For example, when wavelength conversion laser beams are used as the light sources, it is possible to improve the wavelength conversion efficiency and enhance the electro-optic efficiency by making a peak value of a fundamental wave higher with pulse oscillations.
In a light modulating device disclosed in Japanese Patent Application Laid-open No. H11-305710, laser beams corresponding to the colors are modulated by bringing pulse oscillation timing of the color laser beams into synchronization with pixel switching timing of a light modulation element, the color laser beams being oscillated from a red pulse laser, a green pulse laser, and a blue pulse laser, respectively.
The conventional technique described above, however, has a problem where, to reproduce an accurate gradation by using a combination of the pulse oscillation laser and the digital gradation display method, it is necessary to accurately control the section periods for the spatial modulation element as well as the laser beam emission timing and the pulse width of the plurality of laser modules. In addition, there is a limit to the switching period for mechanically driving the spatial modulation element. Thus, it is difficult to control the driving of the spatial modulation element in accordance with the laser beams that are pulse-oscillated with an arbitrary pulse width. Thus, the problem arises where it is difficult to achieve high image quality in the gradation display by increasing the number of sections (hereinafter, “period division number”) into which the time period is divided.